This invention relates to devices for indicating proper installation of a fitting or fastener and more particularly to a mechanical stress indicator that visually indicates whether fastener or fitting is less than optimal, optimal or overtight.
The invention is especially applicable to fittings which rely on swaging to connect a tube to a male or female thread. Such fittings, a type of which, for example, is sold under the trademark Swagelok.RTM., include a male connector, a nut and two internal annular front and back ferrules which receive a tube. The male connector is externally threaded and has an internally tapered mouth that complements the outside taper of the front ferrule. The nut, which accommodates the back ferrule, is partially threaded and has a reduced unthreaded opening that accommodates the tube.
When the nut is tightened onto the male connector it causes the back ferrule to force the tapered front ferrule into the tapered mouth of the male connector. This forcing action compresses the tapered ferrule against the outside diameter of a preinstalled tube thereby binding the fitting to the tube to the extent that the tube yields circumferentially thus providing a mechanical seal. The term swaging, as used herein, is intended to mean the yielding of a tube circumferentially by a conical ferrule in a compression-type fitting.
If the tightening torque between the male connector and the nut is less than optimal the amount of yielding or swaging is less than optimal and the seal between the tube and the fitting will leak. If the tightening torque between the male connector and the nut is too tight, the tube and/or the front ferrule can excessively deform in a manner that prevents a leaktight seal between the tube and the fitting. Excessive deformation of the tube and/or ferrule can render subsequent removal of the tube/ferrule combination from the fitting difficult or impossible. When the tightening torque between the nut and the male connector is at an optimum level, the amount of swaging between the ferrule and the tube is optimum and the fitting parts cooperate to provide a leaktight seal around the tube, and between the ferrule and the nut.
Since a fitting normally has no built-in stress gauges, torque indicators or other means of assessing the appropriate amount of swaging, it is often difficult to recognize when a condition of optimum tightness is achieved. Although a torque wrench can measure the tightening torque applied to a fitting, such wrench may not be available at a time of need or may be inconvenient to use because of space requirements. Also, the use of torque measurements as a means of assessing the proper swaging of the tube/ferrule combination can unduly complicate an installation procedure.
One known installation procedure prescribed by a fitting manufacturer suggests initially tightening the fitting to a condition of finger tightness, scribing the nut at a 6 o'clock position and tightening the nut 11/4 turns. The problem with such instructions is that finger tightness is an arbitrary condition and a scribing tool may not be available to scribe a nut when a fitting is being installed.
Furthermore, even if the manufacturers' installation instructions are followed, the tightness of a fitting can be inconsistent because of discretionary interpretations of finger-tightness and 11/4 turns. Unfortunately, if a fitting is overtightened the tube and the ferrules which help provide the seal can be damaged. Such damage is not visually apparent, and once a fitting has been damaged because of overtightness, the damage cannot be undone.
Another known method of tightening a fitting to a condition of optimum torque includes use of a limit gauge that fits between the nut and the male connector. However, gauges are often unavailable or improperly used. Furthermore, if a fitting is overtightened and then loosened to satisfy a gauge measurement, any damage that may have been caused by overtightness cannot be cured by a subsequent torque reduction.
Visual inspection of an installed fitting will not indicate whether the tightening torque is too loose, too tight or just right. Visual inspections also do not provide adequate corroboration that a fitting is leaktight, or that proper tightening procedures were followed.
Stress indicators used in combination with fasteners are helpful for installers and inspectors if such indicators provide a visual indication of stress or torque that is applied to the fastener. Some known devices for indicating the stress condition or tightness of a mechanical component or fastener are shown in U.S. Pat. Nos. 2,824,481; 3,153,974; 3,224,316; 3,304,827; 3,474,701; 3,589,234; 4,170,163 and 4,636,120. However, such devices are generally built into the fastener and are not economical or feasible for use with a tube fitting of the type described.
It is thus desirable to provide a compact, simply structured mechanical stress indicator for a fastener or fitting that visually indicates, while an installation torque is being applied to the fastener or fitting and after the torque has been applied, whether the resulting installation or swage is less than optimal, optimal or overtight.